1. Field of the Invention
The present invention relates to a viscous fluid type heat generator in which a viscous fluid is subjected to a shearing stress in a heat generating chamber to generate heat that is in turn transmitted to a heat-transfer or heat-exchange fluid circulating in a heat receiving chamber to be carried by the heat-transfer fluid to an area to be heated. More particularly, the present invention relates to a viscous-fluid type heat generator which includes an additional chamber communicated with the heat generating chamber for accommodating a viscous fluid, the amount of which is larger than the capacity of a fluid holding gap defined in the heat generating chamber. The present invention may be embodied, for example, as a supplementary heat source incorporated in a vehicle heating system for comfortably heating the passenger compartment of a vehicle.
2. Description of the Related Art
Japanese Unexamined Utility Model Publication (Kokai) No. 3-98107 (JP-U-3-98107) discloses a conventional viscous fluid type heat generator incorporated in a vehicle heating system, which includes means for adjusting the heat generating performance of the heat generator. The viscous fluid type heat generator disclosed in JP-U-3-98107 includes a housing having mutually opposing front and rear portions which define an inner heat generating chamber and a heat receiving chamber arranged to surround the heat generating chamber. The heat generating chamber is separated from the heat receiving chamber by a partition wall through which heat is exchanged between a viscous fluid in the heat generating chamber and a heat-exchange fluid in the heat receiving chamber. The heat-exchange fluid is introduced into the heat receiving chamber from an external heating system, and is delivered from the heat receiving chamber to the heating system, so as to be constantly circulated through the heat generator and the heating system.
A drive shaft is rotatably supported in the front and rear portions of the housing by bearings, and a rotor element is fixedly mounted on the shaft to rotate inside the heat generating chamber together with the shaft. The rotor element includes outer faces arranged in a face-to-face relationship with the inner wall surfaces of the heat generating chamber to define therebetween small gaps in the shape of axial labyrinth channels. The viscous fluid, generally a polymeric material such as silicone oil which has a high viscosity, is supplied into the heat generating chamber to fill the small gaps between the outer faces of the rotor element and the inner wall surfaces of the heat generating chamber.
The viscous fluid type heat generator of JP-U-3-98107 also includes a viscous fluid reservoir, the casing of which is fixedly attached to the bottom of the generator housing. A diaphragm is supported on the upper inner wall of the reservoir to define therein an additional chamber which is in fluidic communication with the heat generating chamber to permit the viscous fluid to flow freely from one chamber to the other. The heat generating chamber communicates with the environmental atmosphere through a hole penetrating the top wall of the generator housing, thereby allowing the free flow of the viscous fluid. The diaphragm is selectively shifted between uppermost and lowermost positions by the interaction of a manifold negative pressure and a spring force, both applied onto the back side the diaphragm, to adjust the capacity of the additional chamber.
When the drive shaft of the above viscous fluid type heat generator, incorporated in the vehicle heating system, is driven by a vehicle engine, the rotor element is also rotated within the heat generating chamber. At this time, if the diaphragm is located at the uppermost position and thus the viscous fluid entirely fills the heat generating chamber, the rotating rotor element provides a shearing stress to the viscous fluid held between the inner wall surfaces of the heat generating chamber and the outer faces of the rotor element. The viscous fluid then generates heat due to the shearing stress applied thereto. The generated heat is transmitted from the viscous fluid to the heat-exchange fluid circulating through the heat receiving chamber, and the heat-exchange fluid carries the transmitted heat to the heating circuit of the vehicle heating system.
In the viscous fluid type heat generator of JP-U-3-98107, if too much heat is generated by the generator and should be reduced or stopped, the diaphragm is shifted toward the lowermost position by applying the manifold negative pressure onto the back side of the diaphragm, whereby transferring the viscous fluid from the heat generating chamber into the additional chamber of the reservoir. Consequently, the heat generation due to the shearing stress applied to the viscous fluid is reduced or stopped, and the heating capacity of the vehicle heating system is reduced. On the contrary, if the heat generation of the generator is too little and should be increased, the diaphragm is shifted toward the uppermost position by applying the spring force onto the back side of the diaphragm, whereby transferring the viscous fluid from the additional chamber of the reservoir into the heat generating chamber. Consequently, the heat generation due to the shearing stress applied to the viscous fluid is increased, and the heating capacity of the vehicle heating system is increased.
In the above viscous fluid type heat generator, however, when the viscous fluid is transferred from the heat generating chamber into the additional chamber, fresh environmental air is introduced into the heat generating chamber through the top hole of the housing to compensate for the negative pressure in the heat generating chamber due to the removal of the viscous fluid. Therefore, the viscous fluid comes into contact with the introduced fresh air whenever the viscous fluid is transferred into the additional chamber, i.e., whenever the heat generation is to be decreased. This causes problems in that the oxidation and degradation of the viscous fluid is accelerated, and that the viscosity of the viscous fluid is affected or decreased due to the addition of water from the atmosphere.
The above problems caused due to the fresh air introduced into the heat generating chamber can be eliminated by forming the heat generating chamber as a fluid-tight chamber. Viscous fluid type heat generators including such a fluid-tight heat generating chamber are well known in the art, and one example is disclosed in the specification of Japanese Patent Application No. 7-217035 that is a co-pending application by the applicant of the present case. The fluid-tight heat generating chamber does not allow the environmental fresh air to enter therein, and can thus prevent the viscous fluid held therein from coming into contact with the fresh air. Therefore, oxidation and degradation of the viscous fluid is prevented, and the addition of water, from the atmosphere, to the viscous fluid is avoided.
In this type of heat generator including the fluid-tight heat generating chamber, when a rotor element is rotating in the heat generating chamber, the viscous fluid such as silicone oil tends to be collected in the radially inner or center region of the heat generating chamber, by the Weissenberg effect as a normal stress effect caused due to the rotation of the rotor element positioned perpendicularly to the fluid level surface of the viscous fluid. At the same time, the viscous fluid in the heat generating chamber is subjected to the centrifugal force acting radially outwardly from the center region.
In this respect, it has been found that, when the rotor element rotates at a speed lower than a predetermined rotation speed, the Weissenberg effect is much stronger than the centrifugal force, and thus the viscous fluid circulates in the heat generating chamber substantially under the Weissenberg effect. As the speed of the rotor element increases from such a lower speed, the effect of the centrifugal force is increased and the Weissenberg effect is reduced, and thus the viscous fluid circulates in the heat generating chamber under both the centrifugal force and the Weissenberg effect. Then, when the speed of the rotor element exceeds a predetermined high rotation speed, the centrifugal force becomes much stronger than the Weissenberg effect, and thus the viscous fluid circulates in the heat generating chamber substantially under the centrifugal force.
However, the viscous fluid type heat generator including a fluid-tight heat generating chamber has a problem that the viscous fluid is held in very small gaps between the inner wall surfaces of the heat generating chamber and the outer faces of the rotor element to ensure the sufficient heat generation, and thus the viscous fluid is difficult to smoothly circulate in the small gaps. Consequently, the temperature of the viscous fluid in the fluid-tight heat generating chamber, especially in the radially outer regions of the small gaps where the viscous fluid is subjected to the higher circumferential speed of the rotor, rises to a significant level, and the viscous fluid is degraded when the temperature exceeds the limit of the heat resisting properties of the viscous fluid. Therefore, it is difficult, in this type of heat generator, to maintain good, stable and efficient heat generation for a long period.